Microparticles, microspheres, and microcapsules, referred to herein collectively as "microparticles" are solid particles having a diameter of less than one millimeter, more preferably less than 100 microns, which can be formed of a variety of materials, including synthetic polymers, proteins, and polysaccharides. Microparticles have been used in many different applications, primarily separations, diagnostics, and drug delivery.
The most well known examples of microparticles used in separations techniques are those which are formed of polymers of either synthetic or protein origin, such as polyacrylamide, hydroxyapatite or agarose, which are used to separate molecules such as proteins based on molecular weight and/or ionic charge, or by interaction with molecules chemically coupled to the microparticles.
In the diagnostic area, microparticles are most frequently used in the form of a microparticle which serves to immobilize an enzyme, substrate for the enzyme, or labelled antibody, which is then interacted with a molecule to be detected, either directly or indirectly.
In the controlled drug delivery area, microparticles are formed in mixture with molecules to be encapsulated within the microparticles, for subsequent release. A number of different techniques are routinely used to make these microparticles from synthetic polymers, natural polymers, proteins and polysaccharides, including phase separation, solvent evaporation, emulsification, and spray drying.
Microparticles may also be created as a by-product of separations technology, for example, in some precipitation processes, such as precipitation with ammonium sulfate. However, in these cases, the precipitate is collected and compacted by centrifugation and/or filtration, then redissolved in a solvent to separate out the precipitating agent, the salt, from the molecule precipitated with the salt. Accordingly, the microparticles are unstable and function solely as an intermediate product, not as the end product per se.
Spherical beads or particles have been commercially available as a tool for biochemists for many years. For example, antibodies are often conjugated to beads to create relatively large particles specific for particular ligands. The large antibody-coated particles are routinely used to crosslink receptors on the surface of a cell for cellular activation, are bound to a solid phase for immunoaffinity purification, or are used to deliver a therapeutic agent that is slowly released over time at a distant site, using tissue or tumor-specific antibodies conjugated to the particles to target the agent to the desired site.
The most common method of covalently binding an antibody to a solid phase matrix is to activate a bead with a chemical conjugation agent and then bind the antibody to the activated bead. The use of a synthetic polymeric bead rather than a protein molecule allows the use of much harsher activation conditions than many proteins can sustain, is relatively inexpensive, and often yields a linkage that is stable to a wide range of denaturing conditions. A number of activated beads are commercially available, all with various constituents and sizes. Beads formed from synthetic polymers such as polyacrylamide, polyacrylic, polystyrene, or latex are commercially available from numerous sources such as Bio-Rad Laboratories, Richmond, Calif. and LKB Produkter, Stockholm, Sweden. Bead formed from natural macromolecules and particles such as agarose, crosslinked agarose, globulin, deoxyribose nucleic acid, and liposomes are commercially available from sources such as Bio-Rad Laboratories, Richmond, Calif.; Pharmacia, Piscataway, N.Y.; and IBF (France). Beads formed from copolymers of polyacrylamide and agarose are commercially available from sources such as IBF and Pharmacia. Magnetic beads are commercially available from sources such as Dynal Inc., Great Neck, N.Y.
As the wide variety of materials and applications indicates, there is an on-going need for development of new methods for making and using microparticles, particularly those that can be adapted for use in the separations, diagnostic and drug delivery area, rather than in just one application.
It is therefore an object of the present invention to provide stable microparticles and a process for making the microparticles that is relatively simple, rapid, and inexpensive.
It is a further object of the present invention to provide microparticles that have a high affinity and specificity for a target molecule.
It is a further object of the present invention to provide microparticles that are not absorbed when administered in vivo.
It is a further object of the present invention to provide microparticles for use in separations techniques, especially affinity chromatography.
It is a further object of the present invention to provide microparticles for use in medical and diagnostic applications, such as target-specific drug delivery and histopathological or in vivo tissue or tumor imaging.